The present invention relates to the art of disazo condensation pigments. Disazo condensation pigments are compounds formed from two monoazo compounds which are linked by an aromatic diaminocarboxamide bridge. Disazo condensation pigments are traditionally produced in industry in a multistep batch process in tank reactors by mixing the reactants together in an aqueous phase or in solvents depending on the reaction step.
An alternative process comprises continuously mixing the suspensions or solutions of aqueous alkaline coupling component with aqueous acidic diazo component in mixing nozzles (DE-A-1 644 117, DE-A-1 544 453) as final reaction step.
The principal steps in the preparation of disazo condensation pigments are the diazotization of aromatic or hetaromatic amines, the azo coupling reaction and, typical of this class of pigments, the acyl chloride formation of aromatic mono- or dicarboxylic acids with subsequent condensation with aromatic mono- or diamines. The order of the reaction steps can vary. For instance, a diazotized aromatic amine and 2-hydroxynaphthalene-3-carboxylic acid may be coupled to obtain a monoazocarboxylic acid which, after conversion into the carbonyl chloride, is condensed with a diamine. Or two equivalents of a diazotized aromatic aminocarboxylic acid are coupled with a bis-coupler to obtain a disazodicarboxylic acid which, after conversion to the dicarbonyl chloride, is condensed with two equivalents of an aromatic amine. A further variation is the condensation of two equivalents of 2-hydroxynaphthalene-3-carbonyl chloride with an aromatic diamine and subsequent coupling with diazotized amine to form the disazo condensation pigment.
Disazo condensation pigments may need an aftertreatment in water/solvents to obtain the desired crystal properties.
In all these processes, it is essential that the process parameters, such as temperature, time and especially the degree of mixing, be controlled, if the products are to be pure and of high and consistent quality. One problem with these processes is taking new products from the laboratory scale to the large industrial scale.
It is an object of the present invention to provide a process for preparing disazo condensation pigments that provides optimal control of the process parameters, ensures improved mixing of the reactants and simplifies scaleup.
DD 246 257 A1 discloses the possibility of using miniaturized apparatuses for chemical reactions where the chemical entities to be treated are only available in small quantities or are very costly, so that large dead spaces in the equipment become unaffordable.
DE 3 926 466 C2 describes highly exothermic chemical reactions of two chemical entities in a microreactor.
Microreactors for conducting chemical reactions are constructed from stacks of grooved plates and are described in DE 39 26 466 C2 and U.S. Pat. No. 5,534,328. It is pointed out in U.S. Pat. No. 5,811,062 that microchannel reactors are preferably used for reactions that do not require or produce materials or solids that would clog the microchannels.
It has now been found that, surprisingly, microreactors are useful for effecting the diazotization, the azo coupling, the carbonyl chloride formation and the condensation reaction of the carbonyl chloride with an amine to prepare disazo condensation pigments.
As used herein, the term xe2x80x9cmicroreactorxe2x80x9d comprehends micro- and minireactors, which differ only by reason of the dimensions and construction of the reaction channel structures.
It is possible to use, for example, microreactors as known from the cited references or from publications of the Institut fxc3xcr Mikrotechnik Mainz GmbH, Germany, or else commercially available microreactors, for example Selecto(trademark) (based on Cytos(trademark)) from Cellular Process Chemistry GmbH, Frankfurt/Main.
The invention accordingly provides a process for preparing disazo condensation pigments by diazotization of an aromatic amine, azo coupling with a coupling component to form an azocarboxylic acid or azodicarboxylic acid, formation of an azocarbonyl chloride or azodicarbonyl dichloride and condensation of the azocarbonyl chloride with an aromatic diamine or of the azodicarbonyl dichloride with an aromatic amine, which comprises effecting the acyl chloride formation and/or the condensation and optionally the diazotization and optionally the azo coupling in a microreactor.
Advantageously, the starting materials dissolved or suspended in a solvent are preferably fed to the microreactor continuously and preferably in equivalent or, if appropriate, doubly equivalent amounts. Not only reaction catalysts but also classic pigment preparation process assistants, such as resins, surfactants and other additives may likewise be used in the process of the invention.